0 Assigned ID: 2228 ----- Attempting Submission 1 (Fri Apr 09 20:43:54 GMT 2021) ----- HST Phase I Proposal 2228 successfully submitted. Receipt: # 2228-1 ----- Attempting Submission 2 (Fri Apr 09 22:38:42 GMT 2021) ----- HST Phase I Proposal 2228 successfully submitted. Receipt: # 2228-2 ----- Attempting Submission 3 (Fri Apr 09 23:47:31 GMT 2021) ----- HST Phase I Proposal 2228 successfully submitted. Receipt: # 2228-3 ----- Attempting Submission 4 (Fri Apr 09 23:56:56 GMT 2021) ----- The post FLASH level has been adapted from 10 to 14 -212361683 ----- Attempting Submission 1 (Wed Jul 28 08:34:08 GMT 2021) ----- ----- Attempting Submission 2 (Mon Feb 07 15:13:32 GMT 2022) ----- The tension among recent early- and late-Universe determinations of Hubble's constant (H0) is one of the potentially most consequential open problems in astrophysics. Specifically, the present-day measurement based on a well calibrated empirical distance ladder differs by 8.3 +/- 2.0% (at 4.2 sigma) from the cosmology-dependent H0 value derived from the Planck mission's observations of the Cosmic Microwave Background. It is now crucial to demonstrate the accuracy of the distance ladder by scrutinizing even presumably small systematic errors to either resolve or intensify the tension, whereby the latter could place fundamental physics at the verge of a breakthrough. We propose to exploit HST/WFC3's unique UV capabilities to validate and improve the late-Universe H0 measurement based on Cepheid variable stars and type-Ia supernovae (SNeIa). WFC3/UVIS F300X observations of Cepheids in the crucial anchor galaxy NGC4258 will quantify the occurrence of Cepheids in or near their natal (cluster) environments. This will allow us to improve the stellar association bias correction currently applied to the distance ladder by a) comparing clustering frequency, type, and magnitude among all distance ladder anchor galaxies (NGC4258, Milky Way, LMC); b) comparing Cepheid clustering properties between NGC4258 and the SNeIa host M101 under nearly identical observing conditions; c) increasing clustering statistics to investigate their dependence on Cepheid properties. Coordinated parallel observations of a new halo field will accurately calibrate the tip of the red giant branch (TRGB) luminosity and improve the TRGB's ability to serve as a Cepheid-independent first distance ladder rung. We propose deep HST/WFC3 UV imaging in the broad-band F300X passband targeting the field containing > 100 of NGC 4258's known Cepheids, cf. Fig. 3. HST's unique ability to provide deep UV imaging is essential to this project and F300X is the most suitable passband, providing sufficient depth to discover even low-mass, low-luminosity associations that would require a much larger time investment in other UV passbands. The immediate objectives of these primary observations include: 1. conducting anchor-to-anchor and anchor-to-host comparisons of Cepheid clustering statistics as explained in the Scientific Justification above; 2. increasing the sample of known clustered Cepheids beyond the Local Group to measure the mean brightening due to clustering among CDL constituents, investigate its dependence on Cepheid properties (e.g. pulsation period, color, amplitude) and to add diversity to the galaxy environments sampled; 3. optimizing Cepheid selection criteria to minimize H0 bias due to physical association; 4. quantifying the contribution of clustering to the broader problem of crowding; Hence, the requesetd observations will significantly improve the understanding of how Cepheids associate with their natal environments and the ability to correct the CDL-based H0 measurement from stellar association bias. We further propose coordinated parallel ACS/F606W and ACS/F814W observations whose immediate objective is to calibrate the absolute magnitude of the TRGB in a previously unobserved field in NGC4258's Western halo. This is will allow to test conflicting results based on an Eastern field and to understand possible variations in apparent TRGB magnitude. The deep imaging of this field is likely to find future application for the calibration of Mira stars as distance tracers in the JWST era (Huang et al. 2019). Choice of F300X photometric band and exposure time estimation The F300X passband on WFC3/UVIS is best suited for this project because its significantly higher throughput compared to F275W or F336W leads to better detectability of low-mass young stellar populations. Its cut-off at the Balmer jump renders it furthermore less sensitive to the Cepheids than F336W or F390W. We are particularly interested in low-mass clusters and dispersed young associations that are not sufficiently bright to lead to changes in light curve morphology or amplitude, hence the request for deep UV observations. NBODY simulations show that the minimum total cluster mass to form a >10 d Cepheid is approximately 150 -200Msun (F. Dinnbier, private communication). To estimate suitable exposure times, we have estimated the impact of coeval clusters on Cepheid colors, amplitudes, and mean magnitudes using stellar isochrones and synthetic populations computed in HST/WFC3 filter passbands via the CMD 3.2 web interface (Chen et al. 2015). All isochrones and populations are computed assuming solar metallicity and for three ages (30, 50, 100 Myr), representative of Cepheids with three different periods (3, 8, 17d) assuming period-age relations from Bono et al. (2005). To determine the effects of blending on Cepheid properties, we sum the fluxes of one Cepheid (where isochrone crosses instability strip) with the coeval cluster population composed of Main Sequence stars only (none of the randomly populated synthetic populations with M < 2000Msun contained evolved stars). For a 10d Cepheid in a 200 Msun cluster, we find an F300X apparent magnitude of mF300X ~ 24 mag at NGC 4258's distance. Using the HST ETC, we estimate that we can detect such low-mass clusters with a signal-to-noise ratio of > 3.2 in a total of 10210 s (4 orbits) for such a low-mass cluster with a diameter typical of open clusters 0.215" (8 pc at 7.6 Mpc). Cluster populations will be more than 3 mag brighter than the Cepheid in this case, ensuring that the Cepheid would be subdominant. Another smoking gun for an association with young stars will be the blue F300X - F555W color near 0mag or bluer. As the bottom panel shows, such a low-mass cluster will brighten the apparent magnitude by ~0.03 mag, leading to a 1.5% distance bias for such an individual example. To suppress Cepheid detectability at phases near maximum UV brightness, we propose to spread the observations across two visits comprising two orbits each, separated by at least 5 days per visit (total of 4 orbits). The same strategy was adopted in GO-15946 targeting F300X obserations of M101. Each visit will be structured as a WFC3-UVIS-DITHER-BOX pattern in order to better sample the PSF and for cosmic ray rejection. PI Dr. Richard I. Anderson has extensive experience in investigating classical Cepheid variable stars using observations (spectroscopy, photometry, optical interferometry, Gaia) and stellar evolution models (specifically concerning the effects of rotation). Dr. Anderson has contributed to the SH0ES project (PI Adam Riess) on several occasions and led the HST/WFC3 based recent study by Anderson & Riess (2018) that examined the effect of stellar association bias as it applies to the measurement of H0 in the SH0ES project. He also leads HST-GO-15946 that examines stellar association bias in the closest SN-host galaxy M101. Co-I Prof. Adam Riess initiated and leads the SH0ES project that has improved the accuracy of the distance ladder-based Hubble constant measurement by a factor of 5 compared to the Hubble key project. Prof. Riess is also a WFC3 Instrument Scientist and has led (and in many cases carried out in person) a large number of improvements in the analysis and characterization of HST data, both for SH0ES and for the general community. Co-I Dr. Stefano Casertano has participated in the SH0ES project since the beginning and has been responsible for many of the improvements that have been implemented to improve the accuracy of the H0 measurement. Dr. Casertano is an experienced HST user, has been Instrument Scientist and Data Quality Project Scientist for HST, and has worked deeply with WFC3/UVIS data (e.g. to measure Cepheid parallaxes using the spatial scanning mode on HST/WFC3). Co-I Dr. Zoi Spetsieri has expertise in HST data reduction for the WFC3/UVIS, instruments used in these proposed observations. She is an expert on photometric HST data reduction, stellar variability, catalogs and wide-field time domain surveys. In the past she has performed photometry in crowded fields up to 21 Mpc identifying more than 200 variable stars using various variability indexes. 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